1. Field of the Invention
This invention relates to laminated glass articles, more particularly, to bilayer laminated safety glass comprising a polycarbonate urethane adhered to a sheet of glass.
2. Brief Description of the Prior Art
Safety glass is a well-known term for a glass sandwich composed of a plastic interlayer bonding together two glass plates or sheets so that the breaking of the glass results in minimum dispersion of fragments of broken glass. Laminated safety glass is widely used in automobiles and must possess a number of properties including: (1) high impact energy absorption to minimize concussive injury; (2) shear and tear strength sufficient to prevent rupture of the interlayer by broken glass; (3) sufficient adhesion between the interlayer and the glass to prevent dispersion of broken glass to minimize lacerative injury; and (4) good optical quality.
One difficulty encountered in present windshields is that upon breaking, sharp glass edges develop which can result in severe laceration. Bilayer windshields comprising an outer ply of glass and an inner ply of plastic have been suggested for minimizing lacerative injury. However, since the plastic layer will be exposed to the atmosphere, it must have, in addition to the properties enumerated above, excellent weathering properties, such as ultraviolet light and hydrolytic stability, in order to retain its desirable mechanical properties and optical quality.
Since about 1930, plasticized polyvinyl butyral has been the most widely used interlayer material. However, it has been used almost exclusively in conventional trilayer windshields. Polyvinyl butyral is particularly susceptible to moisture and is not amenable to fabrication in the form of a bilayer windshield in which one surface of the polyvinyl butyral is exposed to the atmosphere.
There has been a trend in recent years to provide a substitute plastic interlayers material for polyvinyl butyral. U.S. Pat. Nos. 3,388,032; 3,522,142; and 3,620,950 and Belgian Patent No. 785,125 all disclose various polyurethane materials for use in fabrication motor vehicle safety glass. The polyurethane materials, in general, offer improved physical properties over the polyvinyl butyral in that they have better impact resistance at high temperatures and are not nearly so moisture sensitive. However, a disadvantage associated with the use of polyurethane materials is that many polyurethane formulations "bloom" that is, develop a hazy surface upon exposure to the atmosphere. This "bloom" reduces the optical quality of such materials and discourages their employment in bilayer configurations.
U.S. Pat. No. 3,764,457 entitled "Transparent Impact-Resistant Poly(Carbonate-Urethane) Laminate" to Chang et al. and assigned to PPG Industries, Inc., the assignee of the instant invention, discloses that polycarbonate urethanes, particularly polyoxyalkylenecarbonate urethanes, are useful as interlayers for safety glass windshields. However, polyoxyalkylenecarbonate glycols are very sensitive to ultraviolet light, which curtails their use in bilayer laminates.